1. Field of the Invention
The present invention relates to an air assist system of an engine for enhancing dispersion and atomization of fuel by injecting a part of intake air (assist air) toward the vicinity of a fuel injection nozzle and more particularly to a diagnostic apparatus for the air assist system.
2. Discussion of Background Arts
In automobile engines, a so-called xe2x80x9cassist air supply systemxe2x80x9d, in which a part of intake air (assist air) is introduced from the intake air passage upstream of a throttle valve to the vicinity of an injection nozzle of a fuel injector and injected from an air assist injector so as to enhance dispersion and atomization of the fuel injected from the fuel injector and to improve fuel economy and exhaust emissions, has been broadly employed. As an example of the assist air system, there is a known assist air supply apparatus in which the assist air is introduced during idling from the upstream side of an idle speed control valve (hereinafter referred to as ISC valve) for adjusting the intake air at idling and the amount of the assist air is controlled by an assist air solenoid valve (hereinafter referred to as AAI solenoid valve) provided at the assist air passage.
In such an assist air supply system, the clogging of the assist air passage, the disconnection of the assist air pipe, failures of the AAI solenoid valve and the like are serious failures and objects to be diagnosed. For example, Japanese Patent Applications Laid-open No. Toku-Kai-Hei 8-14143 and No. Toku-Kai-Hei 8-14144 propose diagnostic apparatuses for making a diagnosis of those failures.
In the disclosure Toku-Kai-Hei 8-14143, when the time-versus rate of change of the throttle opening angle is smaller than a predetermined value and in a steady state, a diagnosis is performed. That is, the electromagnetic valve (AAI solenoid valve) in the assist air passage is forcedly energized and deenergized to judge whether or not the assist air supply system is normal based on the change of output of an air flow meter.
Further, in the disclosure Toku-Kai-Hei 8-14144, while the air-fuel ratio feedback control is carried out, an air-fuel ratio feedback correction coefficient is calculated for respective cases of when the assist air is supplied and when it is shut off to judge whether the assist air supply system is normal or abnormal based the difference between both cases.
However, in the respective prior arts, the diagnosis of the assist air supply system is made by forcedly opening and closing the AAI solenoid valve, while fuel is supplied from the fuel injector. That is, in case of the disclosure Toku-Kai-Hei 14143, when the throttle valve is in an open steady condition, since the diagnosis is performed while fuel is supplied from the fuel injector, the electromagnetic valve is forcedly energized and deenergized. Further, in case of the disclosure Toku-Kai-Hei 8-14144, in order to see the change of the air-fuel ratio feedback correction coefficient, it is impossible to diagnoses without supplying fuel from the fuel injector.
Accordingly, in these diagnostic apparatuses, during the diagnosis of the assist air supply system, the actual air-fuel ratio is subjected to changes as a result of the forced opening and closing of the AAI solenoid valve. That is, when the AAI solenoid valve opens, the air-fuel ratio changes to the lean side and when it closes, the air-fuel ratio changes to the rich side. As a result, the attempt to make a diagnosis exacerbates the exhaust emissions.
In case of making a diagnosis based on the behavior of output of an air-fuel ratio sensor as in Toku-Kai-Hei 8-14144, since the change of the output of the air-fuel ratio sensor is small, the diagnosis is easily affected by other air-fuel ratio changing factors such as canister purging, and as a result erroneous diagnoses are apt to occur.
It is an object of the present invention to provide a diagnostic apparatus capable of diagnosing miscellaneous failures of the air assist supply system, such as clogged assist air passages, disconnected pipes, a stuck AAI solenoid valve and the like without adverse effects on exhaust emissions.
In order to achieve the object, a diagnostic apparatus of an assist air supply system of an engine comprises a difference calculating means for calculating a difference between a first detection value of an intake manifold pressure when an assist air is supplied by controlling an assist air supply control valve in the condition that the engine is in a fuel-cut mode and that a valve opening angle of an idle speed control valve is constant and a second detection value of the intake manifold pressure when the assist air is shut down by controlling the assist air supply control valve in the condition that the engine is in a fuel-cut mode and that a valve opening angle of the idle speed control valve is constant and an abnormality judging means for judging an abnormality of the assist air supply system when the difference between the first detection value and the second detection value is smaller than a specified threshold value.